3.8 volts brown wire at alternator ???

[VetteVet]

Are you checking the voltage drop with the brown wire disconnected at the alternator or where ever the end of the wire is. If it is plugged into the alternator, or any current flow ,then you will get a voltage drop anywhere along the wire.

The problem is you're getting way too much voltage drop at the end of the resistance wire.
12 - 3.8 means an 8.2 drop. Or the current draw is way more than it should be.

Measure the resistance on the B/W wire from the key switch with the alternator plug out and see what you have. I would expect 10 ohms which is what the factory rating is.
The charging light in the dash on the light clusters is rated at 10 ohms so the factory matched that with the resistance wire.

[1929fordtudor]

You nailed it! 10.9 ohms from switch to alternator unplugged. Yes I have been testing the voltage with the plug installed on the alternator. Ok, now we are getting somewhere! I have 12 volts on the brown wire at the alternator with the connector unplugged. So from the switch to the alternator I have 12.67 volts. So what could be causing the voltage drop when I plug the connector into the alternator? So let me guess, this is normal with the plug connected and I've been chasing rabbits?

[1929fordtudor]

On to the fuel gauge now. Gauge tests good and new sending unit by PO. There is no ground to the sending unit and there is no tab or pigtail on the sending unit. Where or how do I connect a ground to the sender if there is no pigtail or tab to connect to?

[1929fordtudor]

Ok, I attached a ground to the sending unit. There was a very wide tab that I trimmed down to accept a connector. Ran the ground to the frame rail. With key off the gauge is around 10 o'clock. With the key on it moves to just barely below empty. There is approximately 1/2 to 2\3 tank of fuel. Gauge checks out perfectly fine. Could it possibly be the PO purchased and installed the wrong sending unit?

[VetteVet]

Originally Posted by 1929fordtudor

You nailed it! 10.9 ohms from switch to alternator unplugged. Yes I have been testing the voltage with the plug installed on the alternator. Ok, now we are getting somewhere! I have 12 volts on the brown wire at the alternator with the connector unplugged. So from the switch to the alternator I have 12.67 volts. So what could be causing the voltage drop when I plug the connector into the alternator? So let me guess, this is normal with the plug connected and I've been chasing rabbits?

According to Ohm's Law a 10 ohm resistance on a 12 volt circuit would result in a current of 1.2 amps I = V/R or 12/10 equals 1.2
Since you are reading an 8 volt drop with a 10 ohm resistance You must have more resistance or more current flow. I would suspect more current flow which probably means a bad diode in the alternator. Are you positive that the brown wire is connected to the no. 1 and not the no. 2 terminal on the alternator?
Check the diodes by using the meter leads between the terminals and ground and then reversing the leads. You should get continuity one direction and not the other. Include the large output terminal in the test.

Originally Posted by 1929fordtudor

On to the fuel gauge now. Gauge tests good and new sending unit by PO. There is no ground to the sending unit and there is no tab or pigtail on the sending unit. Where or how do I connect a ground to the sender if there is no pigtail or tab to connect to?

The fuel sender is configured to ground to the fuel tank which should be grounded to the cab or frame. Then the sender terminal is connected to the fuel terminal in the fuse panel. If you ground the terminal on the sender you will get the empty reading you're seeing on the gauge. Un ground the terminal on the sender and connect it to the fuse panel instead of to ground and read the gauge. Your Ohm range is 0 to 90 Ohms. Take a reading between the terminal on the sender and the sender body (flange) looking for 45 ohms which would be close to 1/2 tank of gas.

Originally Posted by 1929fordtudor

Ok, I attached a ground to the sending unit. There was a very wide tab that I trimmed down to accept a connector. Ran the ground to the frame rail. With key off the gauge is around 10 o'clock. With the key on it moves to just barely below empty. There is approximately 1/2 to 2\3 tank of fuel. Gauge checks out perfectly fine. Could it possibly be the PO purchased and installed the wrong sending unit?

See Above check.

[1929fordtudor]

I will check the alternator tomorrow. Maybe I got a bad alternator. It is a rebuilt unit from the local auto parts store.
I have the tan wire from the post on the the sender connected to the fuse panel. I connected the ground wire to the tab on the sender flange. Get the same reading with the ground connected or disconnected. It doesn't seem to make a difference. With the key off the gauge needle sits at about 9 o'clock. With the key on it will move up to just below empty. This is with 1/2 tank of fuel.